It is becoming increasingly important to determine and hold constant the concentration of magnetic particle suspensions, since to an increasing extent quantitative inspection procedures are making use of the magnetic particle method, whereas in the past these procedures have been mostly accomplished purely qualitatively. Since it has become the practice to optically scan accumulations of fluorescent magnetic powder particles and to use the luminous intensity of the particles accumulating at the location of a defect to indicate its depth, changes in the concentration of the suspension directly affect the result obtained, namely, reading of the depth of the defect.
Published West German patent application DT -- AS 2 100 013 makes known a process for holding constant the concentration of a fluid bath used in testing metal objects for cracks, breaks or fissures. In this process, the concentration of the bath, which is required as a control variable, is determined by photosensitive detection means. Although this process has a certain degree of success in preventing powder particle impoverishment in the bath, to date it has not been possible to achieve fully satisfactory results, for the reasons which will be now given. When used for inspection purposes magnetic powder suspensions deteriorate to some extent, not only because their concentration is constantly weakened as particles are extracted to accumulate at the leakage flux locations of the items under inspection, but also because increasingly as a function of time, the shells split away from the paticles, which shells bind the fluorescent or luminous pigments to the particles. This means that the luminous intensity of a suspension includes that of luminous particles which are not, or no longer, bound to magnetic powder particles, and non-magnetizable particles are useless for inspection by magnetic techniques. The process referred to above, in which the concentration of a suspension is measured on the basis of the luminous intensity of random particles, gives unambiguous readings only when the particles whose luminous intensity is being measured actually have a magnetizable core. This, however, cannot be fully assured even when the suspension is freshly produced.